Target acquistion (TA)is passed to the missile to lock on the target (TT) to detonation through a homing guidance radar receiver system located in the missle.
The SA-6 missile has a semi-active TT radar system in which the launcher's Command guidance radar 'paints' the target with a narrow beam while the missile passively searches for the reflected radar off the target, thus one form of track via missile.

That looks more like Semi Active Radar Homing to me.
Track via Missile is when the fire control radar also tracks the target via information sent to it from the missile(s). The missile therefore has a datalink which transmits back to the ground station during the engagement.
As I understand it (not 100% certain), TVM enhances ECCM capability for an attacking SAM system by placing passive recievers (ie, the missiles themselves) in the loop - a network. If the defending platform manages to use ECM to decieve the fire control radar on range to target, for example, a triangulation with the attacking missile's angle to target info can get around the ECM.